1. Field of the Invention
The present invention pertains to the field of transparent electrical conductive coatings made of indium tin oxide which are applied to transparent substrates, especially flexible transparent polymeric film substrates. The invention also pertains to the method of making the coated substrates and the use of the coated substrates in electronic devices which require transparent conductive oxide (TCO) films having excellent electrical conductivity, mechanical durability and high transparency. Such electronic devices include liquid crystal displays (LCD), touch panels (TP) and pen entry devices, electroluminescent lamps (EL), personal digital assistants (PDA), organic light emitting devices (OLED), etc.
2. Background Information
Substrates such as flexible transparent polymeric films having a TCO coating thereon are widely used in the above noted devices because these coatings possess high optical transparency, high electrical conductivity and good mechanical stability. Indium oxide-tin oxide (indium-tin oxide commonly referred to as ITO) is often used as the TCO coating.
The indium-tin oxide coating used in this field of technology and in the present invention is a non-stoichiometric n-type semiconductor which exhibits high visible transmittance, low resistance and high infrared reflectance. For this reason, thin films of ITO are commonly used as the TCO coating in the above noted devices. Conventional reactive sputtering from a InSn alloy in an oxygen containing atmosphere (e.g., argon-oxygen atmosphere) is used to apply the ITO film or coating onto the substrate. A conductive ITO film is a partially oxidized mixture of indium and tin and thus the optoelectronic properties of such films are greatly affected by the level of oxygen admitted into the plasma during the deposition. The amount of oxygen in the ITO films used in this field of technology is well known to those skilled in the art.
On one hand, films with too little oxygen exhibit high sheet resistance and low visible transmittance. On the other hand, at a fully reacted state (complete oxidation), one achieves a transparent oxide with very high sheet resistance and high visible light transmittance. The manner of proceeding from a metallic layer to a fully oxidized layer depends on the feedback and control mechanisms employed during reactive deposition processes which are well known to those skilled in the art.
In the production of ITO coatings from an InSn target on a polymeric web (i.e., sheet), the in-situ measurement and control of the oxidation level is of pivotal importance. The traditional constant pressure controlled reactive sputtering of ITO works well and produces films with adequate properties. However, the use of such traditional constant pressure controlled reactive sputtering of ITO does not meet the demands of more stringent and sophisticated applications. This is because reactive sputter deposition of indium-tin oxide (ITO) from an alloy target is an extremely sensitive process. The quality of the deposited ITO is dependent on the ability to maintain a certain constant partial pressure of oxygen in the sputtering zone during the deposition process. Minor changes in the substrate outgassing, pumping speed, target condition or arcing can result in significant changes in the oxidation level of the deposited layer, thus producing an inferior conducting film. When producing sputtered ITO on continuous flexible substrates, on-line monitoring and feedback control in real time is essential for high quality products.
There are various methods in use today which provide for the monitoring and control of the reactant species in the glow discharge. These methods include direct or indirect partial pressure measurements and optical emissions spectroscopy. In particular, such methods include the use of a residual gas analyzer (RGA), an optical gas controller (OGC) or a plasma emission monitor (PEM). Each of the above devices provide a means for monitoring and controlling the amount of oxygen during deposition of the metal. It is well know to those skilled in the art that the amount of oxygen must be kept at a level to produce the aforementioned non-stoichiometric ITO coating which is not completely oxidized. Thus the oxygen atmosphere during deposition must be maintained at an oxygen deficiency to produce the required non-stoichiometric oxide coating.
ITO coated films are conventionally used in a wide range of applications which include touch panel devices. Touch panel devices have two opposing surfaces of the ITO films separated by spacers. Contact is made between the two opposing surfaces when the front surface is depressed by a finger or touch pen. Depending on the type of device, the location of the input is decoded by the electronic interface according to known technology. LCD devices typically include an array of transparent ITO electrodes which define the display segment or pixels to be activated. In EL displays electrical energy is converted to light energy (luminescence). EL displays use a thin film of phosphor sandwiched between dielectric layers that is sandwiched between two electrodes, one of which is ITO. When an AC voltage is applied to any of the electrodes, the phosphor will be excited so that it emits light.
Reliability during continuous operation is a problem associated with these devices. In the touch panel device it has been observed that the electrical resistance increases after continuous cycling. The ITO surfaces crack or fracture at the touch location and these fractures propagate over time to totally disrupt the operation of the device. Countermeasures have been employed in these devices to help prevent the aforementioned cracking problem. In particular, it is known to deposit a thin layer of palladium, platinum, gold or oxides of these metals onto the ITO film to protect the ITO and/or increase the surface adhesion properties of the ITO layer for subsequent processing. Methods to promote crystallization of the ITO surface after deposition have also been used to grow a hard surface. Problems have also been observed during the fabrication of EL lamps. It has been observed that the ITO film in EL lamps becomes delaminated from the polymeric substrate during processing of the phosphor layer. Pretreatment methods have been utilized to increase the surface energy of the substrates during deposition, but with limited success. Other techniques such as applying a thin metallic or oxide layer on top of the ITO has also been tried with good results.
As active metric liquid crystal displays (AMLCD) become the dominant display for portable systems such as pagers, phones and personal digital assistants, ruggedness and impact resistants become highly desirable for handling considerations as well as for flexibility during their fabrication.
It is an objective of the present invention to provide an improved sputtered TCO coating of ITO on a transparent substrate, especially a transparent polymeric film substrate, for use in applications where TCO films are conventionally employed.
It is also an objective of the present invention to improve conventional electronic devices which include a TCO as a component thereof by using the ITO of the present invention as the TCO.
It is also an objective of the present invention to provide a method for making an improved transparent conductive coating of ITO on a transparent substrate.
These and other objectives are obtained by forming a multilayered transparent conductive film or coating of ITO on a transparent substrate wherein the ITO film or coating includes distinct layers of transparent conductive ITO. The term xe2x80x9ctransparent conductive coating of ITOxe2x80x9d as used herein refers to transparent conductive ITO coatings which are conventionally employed in well known electronic devices which require a TCO. Such devices include conventional liquid displays (LCD), touch panels (TP), electroluminescent lamps (EL), personal digital assistants (PDA), organic light emitting devices (OLED), etc. Henceforth such transparent conductive ITO coatings will be referred to herein as ITO coatings. In addition, the term xe2x80x9cconductivexe2x80x9d as used herein refers to electrical conductivity.
The layers of ITO are spatially distinct from each other and differ in terms of the proportion of indium to tin in each layer. Aside from the proportion of indium to tin (i.e., the relative amount of indium and tin on an atomic basis) in each layer, the compositional characteristics of the ITO coatings are otherwise the same as the nonlayered ITO which is conventionally used in TCO materials.
The composition (i.e., relative amount of In and Sn in each layer) is selected in each layer to provide desired properties at varying depths in the ITO coating. It has been discovered that the problems relating to lack of durability and other mechanical and physical disadvantages of the prior art ITO coatings can be overcome by providing the multilayered ITO of the present invention without adversely affecting the optical properties of the ITO film. For example, in the present invention the composition of one or more layers of the ITO film can be selected to provide a chemical resistance barrier which protects the entire structure while the composition of the other ITO layers can be selected to optimize other characteristics without adversely affecting optical properties of the device. The composition of any of the ITO layers can be custom made to provide desired physical characteristics at specified locations on the ITO surface and within the depth of the ITO coating.
The TCO film of the present invention comprises a plurality of ITO layers with at least one ITO layer being different (i.e., having a different In to Sn ratio) from one or more other ITO layers. Thus in its simplest embodiment, a substrate has an inner ITO layer thereon and an outer different ITO layer on the inner ITO layer. Additional ITO layers may be included.
The plurality of layers forms a stack of two or more layers, each layer having a finite thickness wherein the composition or proportion of In to Sn is substantially uniform throughout the thickness thereof. By substantially uniform it is meant that a given layer is as uniform as humanly possible when sputter coated using a homogenous target containing the desired ratio of In to Sn. The uniform composition of each layer exists as a finite thickness, e.g., about at least 50 angstroms, preferably 50-600 angstroms, in each layer. Thus there is no continuous gradient of compositional change throughout the entire thickness of the entire ITO coating. Naturally there may be a zone of nonuniformity between layers due to inherent imperfections in the sputtering process when one thin ITO layer is sputter coated (using one target) onto a different thin ITO layer (using a different target). Such zones may be referred to as transition zones between the ITO layers. The changes in composition through the transition zones may be gradual.
It is possible that two or more ITO layers in a stack containing at least three ITO layers, may be compositionally identical to each other. However, compositionally identical layers are separated from each other by another compositionally different layer.
The substrate coated with the ITO layers may include additional layers which are typically used in TCO devices in this field of technology. Such layers include protective top coat layer, primer layer, hard coat, etc. The substrate may thus include other layers as a component thereof.
Since the above-described multilayered ITO film has compositionally different ITO layers wherein the compositional difference lies in the In to Sn atomic ratio, the ITO coating may be referred to as a graded ITO stack.
The substrate used in the present invention may be any of the commonly employed substrates which are typically used in electronic devices which employ a TCO coating. Such well known substrates include transparent flexible polymeric film or sheets. Suitable polymers for making the polymeric film include polyester such as polyethylene terephthalate (PET), polyurethane, polysulfone, and polycarbonate.
Sputtering is advantageously used to deposit the ITO layers of the TCO film so that high temperatures and other physical and chemical conditions which could harm the substrate, especially a polymeric substrate, can be avoided. Other physical vapor deposition procedures, such as evaporative coating, may be employed. Sputtering is particularly advantageous because it can deposit the ITO layer at ambient or room temperature (e.g., about 70xc2x0 F.). Any conventional sputtering method and apparatus may be used; it being understood that each compositional layer requires the use of a target which has the corresponding proportion of In to Sn. Sputtering coaters which advance a polymeric sheet or web from one roll to another roll with a plurality of sputtering stations positioned in the path of the sheet or web are particularly advantageous because the various layers of ITO can be sputter coated onto the sheet in one pass of the sheet as it advances from one roll to the other roll. Such sputtering coaters are well known and are commercially available. An example of such a sputtering device is shown in U.S. Pat. No. 4,977,013, the specification of which is incorporated herein by reference.
The sputtering target may be an indium-tin alloy. When sputtering with an indium-tin alloy, the sputtering is performed in an atmosphere which contains oxygen according to well known techniques so that the deposited material is the desired oxides of indium and tin. As noted above, the indium tin oxide coating has a nonstoichiometric amount of oxygen so that the coating has the required electrical conductivity and transparency; and the amount of oxygen contained in the ITO coating in the present field of technology is well known to those skilled in the art. Alternatively, instead of using an alloy target in an oxidizing atmosphere, the sputtering may use a tin oxide-indium oxide ceramic (e.g., a mixed ceramic powder of indium oxide and tin oxide) as the target. Such a target should have an oxygen content equal to the oxygen content in the corresponding film or ITO layer which is deposited.
The amount of indium and tin in a particular ITO layer can be expressed in terms of atomic percentage wherein the atomic percent of one of the two metals is in relationship to the total content of those two metals in the particular layer.
Thus an ITO layer having 1% tin will have 99% indium and an ITO layer having 5% tin will have 95% indium. Similarly an ITO layer having 99% tin will have 1% indium and an ITO layer having 95% tin will have 5% indium. The above percentages are on an atomic basis. Unless stated to the contrary, all of the percentages of tin and indium described herein are on an atomic basis as described above.
The present invention is based on the discovery that TCO""s having an atomic percentage of 1-99% tin deposited by sputtering techniques can be improved by stacking ITO layers in the manner described above to form a graded array or stack of the coatings. Each ITO layer of the stack embodies a specific physical property that has been tailored for the desired device which uses the TCO layer. For example the touch input device has a front surface ITO layer which possesses greater tin content than the back surface that is in contact with the polymeric film. This type of structure has enhanced mechanical characteristics for continuous touch input while maintaining excellent bulk conductivity and high transparency comparable to the conductivity and transparency of conventional single composition ITO materials. More specifically, multi-compositional graded ITO structures of the present invention provide for more rugged features which are suitable for a variety of new electronic devices. The multi-compositional ITO also possesses greater environmental stability when exposed to high temperature and humidity conditions.
It has been discovered that the TCO""s of the present invention deposited by sputtering techniques are more durable and possess greater flexibility which is required for touch type devices in comparison to the TCO""s of the prior art.